Polarization entanglement enabled by orthogonally stacked van der Waals NbOCl<sub>2</sub> crystals.

Guo, Qiangbing; Wu, Yun-Kun; Zhang, Di; Zhang, Qiuhong; Guo, Guang-Can; Alù, Andrea; Ren, Xi-Feng; Qiu, Cheng-Wei · Nat Commun · 2024

basic_science · Level V

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Abstract

Polarization entanglement holds significant importance for photonic quantum technologies. Recently emerging subwavelength nonlinear quantum light sources, e.g., GaP and LiNbO<sub>3</sub> thin films, benefiting from the relaxed phase-matching constraints and volume confinement, have shown intriguing properties, such as high-dimensional hyperentanglement and robust entanglement anti-degradation. Van der Waals (vdW) NbOCl<sub>2</sub> crystal, with strong optical nonlinearities, has emerged as a potential candidate for ultrathin quantum light sources. However, polarization entanglement is inaccessible in the NbOCl<sub>2</sub> crystal due to its unfavorable nonlinear susceptibility tensor. Here, by leveraging the twist-stacking degree of freedom inherently in vdW systems, we showcase the preparation of polarization entanglement and quantum Bell states.